This invention relates to turbomolecular pumps, and more particularly it deals with a turbomolecular pump suitable for use in achieving a high degree of compression of gas and a high gas discharge speed.
Generally, an evacuated chamber in which a high vacuum is achieved is necessary for a nuclear fusion system, a semiconductor manufacturing apparatus, an electron microscopic device, etc, and to attain the end of providing an evacuated chamber of high vacuum, it has been proposed to employ a turbomolecular pump which exhibits a high pumping performance in a molecular flow.
There are basically two types of turbomolecular pumps, namely, an axial flow molecular pump and a helical groove molecular pump.
An axial flow molecular pump comprises multi-stage axial flow turbines each comprising rotor blades and stator blades of mirror image configuration which are located symmetrically and alternately in an axial direction. In axial flow molecular pumps, the rotor blades are rotated at high speed to impart a specific direction to gas molecules, to evacuate a space. It is surfaces of the blades of rotors and stators that imparts the direction to the gas molecules, and wall surfaces of the rotors at the bottom of the rotor blades and wall surfaces of a casing facing forward ends of the rotor blades are not concerned in the pumping action.
Although this type of molecular pump offers the advantage that a high pumping speed is obtainable, it suffers the disadvantage in that it has a low compression ratio per stage, thereby making it necessary to arrange blades in a plurality of stages, to obtain a high compression ratio. Thus, when this type of molecular pump is employed, it is the usual practice to employ turbines arranged in ten-odd stages. Difficulty has, therefore, been experienced in achieving a high speed in rotation because of a heavy weight of the rotating mass. Moreover, the use of turbines of a plurality of stages requires a large number of personnel and increases the time of manufacturing. In addition, the need to use a half structure for the row of stators to facilitate an assembly of the pump increases the production cost.
Meanwhile, a helical groove molecular pump comprises a casing, and a rotary inner cylinder and a stationary outer cylinder located in the casing in face-to-face relation to each other, with the outer casing being formed with a helical groove. Rotation of the rotary inner cylinder at high speed causes the surface of the inner cylinder to impart direction to gas molecules, and the gas molecules are guided to flow along the helical groove, to thereby discharge gas. Pumping can be effected on the same principle by forming a helical groove on the surface of the rotary inner cylinder and rotating the inner cylinder within the outer cylinder. The helical groove molecular pump is distinct from the axial flow molecular pump in that, whereas, the pumping action is performed by the surfaces of the blades in the latter, this action is performed by the surface of the inner cylinder facing the helical groove on the outer cylinder in the former.
The helical groove molecular pump is simple in construction and can be readily fabricated. However, in a helical groove type of molecular pump, the pumping speed is reduced as an exponential function of the depth of the helical groove, and thereby restricting the application of the helical groove type of molecular pump to vacuum devices to which the pumping speed is not essential.
An added disadvantage of the helical groove molecular pump is that an increase in the gap between the rotary inner cylinder and the outer cylinder formed with the helical groove results in a sudden decline in performance.
Thus, the axial flow molecular pump is more favored than the helical groove molecular pump except for those applications whose purposes can be better served by the latter. It has been proposed in, for example, Japanese Patent Publication No. 33446/77, to use a compound type turbomolecular pump which avoids the disadvantages of the two types of molecular pumps and utilizes their advantages. However, no one type of turbomolecular pump has ever been successful in solving the above described problems of the prior art.
This invention has as its object the provision of a novel type of turbomolecular pump capable of achieving a high compression ratio and a high pumping speed.
In accordance with the present invention a turbomolecular pump is provided wherein a pumping action is performed by a plurality of grooves on a rotor located in a casing and extending axially thereof, and a plurality of grooves on a stator located in the casing in face-to-face relation to the rotor. The features of the invention include a plurality of rotor grooves on an outer peripheral surface of the rotor extending equidistantly spaced-apart relation to each other and tilting at a predetermined angle with respect to the axis of the rotor, and a plurality of stator grooves on a surface of the stator facing the rotor which extend equidistantly spaced-apart relation to each other and tilt at the same angle as the rotor grooves but are oriented in an opposite direction to the rotor grooves, with the rotor grooves and stator grooves overlapping in part as viewed axially of the rotor. The turbomolecular pump having the above-described features is connected at its discharge side to a vacuum device of a nuclear fusion system or the like, and the rotor is rotated at high speed to allow a pumping action to be performed between the rotor grooves and stator grooves, to produce a high vacuum in the vacuum device. The turbomolecular pump of this construction and operation performs both the pumping action which is performed by the surfaces of the blades in an axial flow molecular pump and the pumping action which is performed by the rotor and the bottom surface of the groove in the helical groove molecular pump, whereby a high compression ratio and a high pumping speed can be obtained.